Two new specimens of Hyaenodon venturae from the early Chadronian (latest Eocene) of Wyoming provide new information about this rare, miniature hyaenodont, and extend the upper end of the temporal range of the species. The significant difference between H. venturae and H. microdon, the two smallest North American species of Hyaenodon, is the absence of synostosis of the mandibular symphysis in adult H. venturae. In Hyaenodon microdon, from the middle Chadronian, on the contrary, fusion of the symphysis is complete while the individual is still in the juvenile stage of ontogenesis, before the eruption of m3. Within the genus Hyaenodon the m1 becomes relatively smaller as the length of the tooth row (p1-m3) increases: this relationship can be demonstrated in the series of species from the miniature H. venturae to the giant H. megaloides.